1. Field of the Invention
The present invention relates to a poly(lactic acid) resin composition for preparing transparent and impact-resistant articles.
2. Description of the Prior Art
With the increasing awareness of environmental protection, numerous manufacturers have gradually replaced the traditional plastics used for food packages or disposable packaging containers with biodegradable plastics, and among the biodegradable plastics, poly(lactic acid) (PLA) resins have the most preferable gloss and transparency. However, the articles made from PLA resin are known to have disadvantages such as high brittleness and poor impact resistance at room or low temperature, and are very likely to crack during transportation due to extrusion and collision. Take the current PLA resin containers widely used for packaging of fresh vegetables or frozen foods as example, if fragments formed by cracking of the containers during transportation fall onto the foods, the degree of customer trust is likely to decrease, and the repurchase rate will go down remarkably. As the brittle crack is caused by poor impact resistant strength of the PLA resin, if this disadvantage is to be eliminated, it is necessary to improve the impact resistant strength of the PLA resin.
It is known that a novel material can be obtained by subjecting two or more organic or inorganic materials having different properties to a blending process, and in this way, the properties of the original different materials can be reinforced in this new material. For example, U.S. Pat. No. 6,943,214 has disclosed a use of a polyoxymethylene (POM) polymer which has a property of low glass transition temperature (Tg). This material is processed with a PLA resin to form a mixture of POM and PLA (toughened polyoxymethylene-poly lactic acid compositions), which is said to improve the impact resistance of the original biodegradable plastic. However, it is known to persons skilled in the art that because the mixture of POM and PLA is an incompatible system, severe phase separation of the system will occur, and the modified PLA resin will become opaque. Moreover, in addition to the influence on the biodegradability of the PLA resin, the addition of the polyoxymethylene polymer only has a limited improving effect on the brittleness and impact resistance of the PLA resin. On the other hand, U.S. Pat. No. 6,495,631 improves the toughness of PLA with an epoxidized rubber, by adding an epoxy modifier capable of reacting with PLA resin to reduce phase separation and thus increase the impact resistance of the PLA resin. However, this technology can significantly reduce the biodegradability and transparency of the PLA resin, and also has a limited improving effect on impact resistance.
In order to improve the impact resistance of the PLA resin while maintaining its biodegradability, U.S. Pat. No. 6,803,443 has disclosed that the esterified product of lactide and a polyester is subjected to a ring-opening reaction by ring-opening polymerization, to form a copolymer (polylactide-co-polyester), which is then admixed into the PLA resin as an impact resistance modifier. Although the most preferred embodiment disclosed in U.S. Pat. No. 6,803,443 can achieve the effect of no crack at the detection limit of the analyzer, this technical method requires polymerization starting from the initial lactide to synthesize an impact-resistant co-polyester of PLA, and needs further blending and extrusion so as to improve the impact resistance of the PLA resin. Therefore, besides involving a process which is too complex to have high industrial applicability, this technical means will also significantly compromise the transparency of the PLA resin, thereby limiting the commercial application fields of the PLA resin.
U.S. Pat. No. 7,160,977 has disclosed that the product blended a soft biodegradable polymer (A) with a hard biodegradable polymer (B). The technique can be divided into four main systems: (1) A/B=aliphatic-aromatic copolyesters (AAPE)/poly(lactic acid) (PLA) or co-polyester plus lactic acid (CPLA); (2) A/B=polycaprolactone (PCL)/polyhydroxybutyrates (PHB); (3) A/B=polyhydroxybutyrate-hydroxyvalerate copolymers (PHBV)/polyglycolide (PGA); and (4) A/B=polybutylene succinate (PBS) or polyethylene succinate (PES) or polybutylene succinate adipate (PBSA)/PLA. Although in this technique, the blending of biodegradable polymers such as AAPE with PLA resins can improve the impact resistance of the resin while maintaining the original biodegradability, it has the disadvantage of compromising the transparency.
U.S. Pat. No. 7,465,770 discloses a method for preparing an environmentally degradable copolymer compound comprising the steps of providing a lactic acid polymer by polycondensating a lactic acid to form the lactic acid polymer and then coupling said lactic acid polymer to a flexibilizing aliphatic polyester under condensation and/or transesterification reaction conditions optionally with a coupling agent, such as silanes. U.S. Pat. No. 7,465,770 discloses a further step of endcapping or chain extending of the polymer chain of the polymer obtained from the above steps to improve the stability of the polymer, increase the molecular weight or to create different functionality or non-functionality, and discloses using among others, silanes, as a chainextender or crosslinker. U.S. Pat. No. 7,465,770 focuses on coupling a poly(lactic acid) with a flexible aliphatic polyester so as to lower the Tg of the poly(lactic acid) and flexibilize the poly(lactic acid) for its suitable use as a hot melt adhesive.
U.S. Pat. No. 7,317,069 discloses a process for producing an aliphatic polyester. U.S. Pat. No. 7,317,069 only discloses the use of a silane as a chain extender to increase the molecular weight.
In the past few years, in addition to blending an exiting polymer material with two or more organic materials or polymers having different properties to improve the mechanical properties of the polymer, attempts have been made to add inorganic reinforcing materials, inorganic powders, or fibers, such as glass fibers, minerals and clays, to a polymer base to improve the strength and impact resistance of the polymer material. However, no matter whether dispersion of the inorganic supplement materials, inorganic powders, or fibers into the polymer substrate is achieved by mechanical blending, melt blending, or solvent blending, to the method involves physical blending. It is understood by persons skilled in the art that because these inorganic additives are generally in the form of powder or slurry, if they are simply added and mechanically stirred, they will be prone to severe secondary aggregation in the polymer material even if they are dispersed, so re-dispersion in the polymer material cannot be effectively achieved. Because the compatibility of the polymer base with the additives is an important factor affecting the mechanical properties of the material, sometimes the technical means not only does not improve the properties but results in deterioration of the properties instead.
For example, both U.S. Pat. No. 6,888,663 and US 2006/252890 teach use of powdered silicon- or ammonium-containing inorganic materials, for example, montmorillonite, smectite, mica, polyhedral oligomeric silsesquioxanes (POSS) and ammonium modified clay, to modify the polymer material, but the modification effects are not obvious due to poor powder dispersion. The bottleneck of the technical means lies in surface treatment and dispersion of inorganic powders. Nano inorganic powders, due to large specific surface area, are prone to agglomeration by secondary bonding (for example, hydrogen bond, electrostatic force, and Van der Waals force) on the surface. Furthermore, depending on the different polymer resins and the different functional improvement purposes to be achieved with inorganic powders, different surface treatment techniques may be needed, which makes the operation difficult and large-scale commercial production/supply impossible.
Some recent techniques teach dispersion of intercalated clay into a polymer base in the form of intercalation or exfoliation by means of admixing. However, the intercalated clay often needs to be treated with various surfactants to reach a certain intercalated distance and allow a polymer to enter between the layers, and desirable condition of complete exfoliation cannot be achieved in current mass production, so the technical means cannot significantly improve the mechanical properties of the polymer, either.
Since 1846, there has been extensive academic research on sol-gel systems (“sol-gel” in abbreviation). The sol-gel refers to a combination of two states where non-metal/metal alkoxide is gradually formed from a liquid state into a colloid state through hydrolyzation, condensation, polymerization and the like, and further into nanoparticles with a porous net-like structure having a large surface area. In general, when the sol-gel process is used to produce glass or ceramics, alkoxide monomer, for example, Si(OR)4, in which R can be CH3, C3H7, and the like, is often used as the precursor. Sol-gel process generally includes three steps, namely, hydrolyzation, condensation and polymerization. In the entire reaction, condensation occurs simultaneously with hydrolyzation, rather than starting after complete hydrolyzation. Furthermore, acid or base can also be added in the reaction as catalyst, and the sol-gels generated under different catalytic conditions will have different structures. In acidic conditions, quick hydrolyzation and slow condensation will occur; therefore, the structure tends to be in the form of long chain, and a net-like structure having low crosslinking degree will be formed. In basic conditions, the hydrolyzation rate is slower than the condensation rate; therefore, the monomer will grow into branched chain, and form an un-uniform colloid particle having high crosslinking degree.
In 2007, Jingo Yin and Xuesi Chen proposed in Journal of Materials Letters that, in the presence of solvent tetrahydrofuran (THF), in addition to the plasticizers, PLA and polyethylene glycol (PEG), tetraethoxysilane (TEOS) be introduced into the sol-gel process; at controlled hydrolyzation rate, the hydrolyzed compound was co-polycondensed in the polymer solution to form a gel, and the mixed solution was placed into an oven at high temperature to remove the residual solvent THF and obtain a PLA-silica nanocomposition. However, the literature only discloses that silicon oxide can be used to improve the tensile strength and thermal stability of the PLA resin, and the process has to be carried out in the presence of a lot of solvent. As known to persons of skill in the art, drying and recovery of a lot of solvent are extremely difficult, thus, this means is not suitable for commercial mass production.
In summary, there still is a need for a technical solution in the industry which caneliminates the disadvantages of the PLA resin such as brittleness and poor impact resistant strength at room/low temperature while maintaining the original biodegradability and transparency of the PLA resin, and which has a simple process flow to facilitate commercial mass production. It is found by the inventors of the present invention through extensive research that the PLA resin composition having the components defined herein can efficiently solve the problem.